This invention relates to electronic ballasts adapted to supply operating power to high-intensity discharge lamps.
The function of an electronic ballast is to supply the power required for starting and then operating a high-intensity discharge (HID) lamp, such as a metal halide lamp. A metal halide lamp is a high-pressure gas discharge lamp in which metal halides are enclosed in a quartz envelope. Because this lamp has a compact geometry and a high efficacy of nearly white light, it is now widely used to illuminate sports stadiums and roadways. This lamp also has many industrial and domestic applications.
To initiate its operation, a metal halide lamp demands a high ignition voltage. But once an arc discharge is ignited, the lamp is thereafter maintained in operation by a voltage no higher than the voltage of the AC power source to which the ballast is connected. Thus the function of an electronic ballast is to supply to the HID lamp with which it is associated the voltages and currents needed to start and then operate the lamp at its rated wattage.
While a metal halide lamp is notable for its compact geometry, this feature is absent in existing electronic ballasts for supplying power to the lamp, for the typical multi-stage electronic ballast has many magnetic and power components, and these cannot be compactly packaged. The concern of the present invention is with the creation of an electronic ballast for an HID lamp which has fewer magnetic and power components than a standard electronic ballast, yet is compact, highly efficient and reliable in operation.
The efficiency of an electronic ballast in supplying power to an HID lamp largely depends on its power factor rating. Power factor is defined as the real input power level divided by the apparent input power level. The apparent power level, expressed in watts, is determined by the RMS voltage value multiplied by the RMS current value. Power factor is a function of the degree to which the load current and voltage are in time phase with each other. The greater the degree to which the load current leads or lags the voltage, the lower is the power factor rating and the less efficient the ballast.
To provide electronic ballasts for HID lamps that have a high power factor rating, there are disclosed in the Weng U.S. Pat. Nos. 6,034,489 (2000) and 5,986,901 (1999), and in the Sun U.S. Pat. No. 6,020,691 (2000) electronic ballasts which include a power factor correction (PFC) stage. Supplied to this stage is unregulated DC power derived from a bridge rectifier connected to an AC power line. This PFC stage acts to bring the input current substantially in phase with the voltage and in doing so imparts a high power factor rating to the ballast.
The PFC stage disclosed in the above-identified U.S. patents is associated with other ballast stages, such as a power control stage to maintain at a desired level the wattage of power supplied to the HID lamp, and a storage capacitor stage to regulate the DC power supplied to the power control stage. In the capacitor stage, the capacitor acts to store the energy when the line voltage goes below the RMS level.
The various magnetic and power components and the transistors functioning as electronic switches which are entailed by the separate stages of the ballast associated with the PFC stage disclosed in the above-identified patents contribute substantially to the size, weight and cost of the ballast. And the relatively large number of these components precludes the creation of a compact electronic ballast capable of operating a compact HID lamp.
In view of the foregoing the main object of this invention is to provide an electronic ballast for an HID lamp characterized by a high power factor rating and high efficiency, yet having fewer magnetic and power components than existing ballasts for the same purpose.
More particularly, an object of this invention is to provide an electronic ballast whose power factor correction circuit, storage capacitor circuit, and power control circuit are integrated into a single circuit having relatively few magnetic and power components as compared to the number of components necessary when these circuits are embodied in separate ballast stages. A significant advantage of an electronic ballast in accordance with the invention is that it can be packaged in a compact, light-weight form. Another advantage of the electronic ballast is that it can be mass-produced at relatively low cost.
Briefly stated, these objects are accomplished by an electronic ballast for supplying operating power to a high-intensity discharge (HID) lamp which includes a full-wave rectifier connected to an AC power line to produce an unregulated, pulsating DC output which is applied to a power factor correction (PFC) circuit. The PFC circuit includes a first semiconductor electronic switch whose periodic activation is controlled to bring the input current and voltage more closely in time-phase with one another, thereby imparting a high power factor rating to the ballast.
The pulsating DC output of the PFC circuit is applied to a storage capacitor circuit which is charged thereby, the capacitor circuit including a second electronic switch whose periodic activation is controlled to so discharge the capacitor as to cause the capacitor circuit to yield a regulated DC output. The regulated DC output is fed to a power control (PC) circuit which includes a third electronic switch whose periodic activation is controlled to maintain the power supplied to the HID lamp at the rated wattage of the lamp. The periodic activations of the first, second and third switches are mutually synchronized.